>From the July 2001 issue of Discover Magazine:
 

  The Discover NeuroQuest column

  Anthropology of the brain

  Replay 525 million years of evolution (then add 40 weeks of gestation for
  good measure)
  By Eric Haseltine

  The catchiest -if not most accurate- phrase in biology is "Ontogeny
recapitulates phylogeny." Haeckel, the 19th century scientist who authored
these famous words thought he'd discovered an important law of nature
whereby the embryonic development (ontogeny) of organisms retraces their evolutionary history
(phylogeny).
Haeckel's recapitulation theory would explain, for example, why developing
humans in the womb acquire, then lose, gill-like pouches and tails.

  Hit the Phylogeny button to observe the different stages of human
evolution from 525 million years ago, when our ancestors were
invertebrates called chordates, to the present. As the chart unfolds from
left to right, you'll see major branch points that represent ancestors we
share with modern vertebrates. We don't know exactly what the brains of
these common ancestors looked like, but we do have modern examples of
vertebrate brains starting with "primitive"(i.e. simple) forms such as
sharks on up through "advanced" (i.e. complex) species such as apes.
 

  Notice that the most pronounced changes from "primitive" to "advanced"
brains are in the size of the cerebrum (shown in lavender), and in the
amount of convolution or folding of cerebral tissue. For example, the
modern mouse brain shown next to the early mammal branch point is both
smaller and smoother than that of monkeys, which in
turn is smaller and less convoluted than that of apes. Although you can't
see it directly from the drawings, convolutions are actually just another
way that the brain packs more gray matter into a confined skull by folding
the cortex in on itself (in the same way that many feet of intestine are
folded to fit inside a small abdominal cavity).

  Now, click on the Ontogeny button to see the different stages of
embryonic human brain development. Human development, by progressively
adding cerebral cortex and  convolutions, mirrors the phylogenetic
sequence of simple to complex vertebrates.  Ontogeny, at first blush, does
seem to recapitulate phylogeny.

  Unfortunately, Haeckel's theory turns out to be more elegant than true.
Species such as the Axolotl (a type of salamander) never go through a developmental stage
of not possessing gills, even though their recent ancestors were gill-less.

  Many Biologists these days attribute similarities between phylogeny and
ontogeny to a phenomenon called heterochrony (literally different time).
According to the heterchrony principle, successful mutations are believed to either speed up the rate
of maturation allowing "terminal addition" of new characteristics (such as bigger
cerebrums), or slow down development, eliminating features that are no
longer adaptive (such as absence of gills in Axolotl ancestors).

  Although human brain phylogeny as suggested by the chart shows a steady
addition of new features, In theory at least, human evolution could take a
U-turn like that of Axolotl, giving new meaning to expressions like "green
around the gills" and "tail
between your legs"

http://www.discover.com/neuroquest/neuroquest.html

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